Mr van Gerwen is also part of the VALK Foundation, a collaborative venture by the University of Leiden, KLM and Schiphol to treat people with a fear of flying.

What you need to do is diagnose what the background complaint is. Then you get a much higher success rate

Study author Mr van Gerwen

As many as four in 10 of us are afraid of flying.

Mr van Gerwen studied 5,000 men and women who had a fear of flying.

By interviewing these people and looking at sociodemographic factors he found four distinct subgroups of phobia.

Subtypes of fear

The least phobic of the four were men aged around 35. Their fear stemmed from not being in control of the plane and were worried about things going wrong.

The second group were women younger than 35 with what Mr van Gerwen called 'social' fears.

They worried about other people and about events such as terrorists hijacking the plane. They were also worried they would lose control of themselves during the flight and shout or scream uncontrollably.

Women aged 32-54 formed the third group. They tended to have an underlying fear of confined spaces, called claustrophobia.

They were most fearful when the airplane doors closed and were worried about being able to get out. They were more likely to have panic attacks while onboard.

You need to take it on a client by client basis

UK psychologist Mr Cliff Arnall

The group with the greatest fear of flying was people older than 54, particularly men.

They had a combination of a fear of heights and a need to stay in control of the airplane, and were also prone to panic attacks.

This group was the most likely to avoid flying altogether.

Mr van Gerwen said he was not surprised to find men and women had different fears but was surprised by the age-related trends.

Age trend

"The older people were more fearful of heights. That really surprised us."

He does not know why different people fear different aspects of flying, but said his findings would help improve the success of flying phobia treatments.

"Fear of flying is a very treatable psychological complaint.

"Some people give a one or two day course to try to get people over their fear. What you need to do is diagnose what the background complaint is. Then you get a much higher success rate," he said.

Treatments can then be tailored to what the individual needs - exposure to flying, relaxation training, information about aviation and/or lessons in how to control distressing thoughts.

Mr Andrew Cunningham, a therapist at the National Phobics Society did not think the classifications would necessarily alter the way a client should be treated.

"It may be absolutely valid to have these distinctions with aspects of fears and age and sex but for me, personally, it's unclear how that would help the client choose how they get help and also if it would be helpful for the therapist."

He said this was because people with a flying phobia often had many different underlying fears of the whole flying process and that therapy often involved a mixture of techniques to tackle these.

Mr Cliff Arnall, a psychologist in Brecon and member of the British Psychological Society, echoed Mr Cunningham's reservations. He added: "You need to take it on a client by client basis."